Electrostatographic machines generally use a two-component developer mixture comprised of a toner powder and a magnetized or magnetizable carrier material. During the use of the machine, toner powder has to be replenished to compensate for its consumption during image development. Various automatic toner replenishment systems are known wherein a signal from a toner concentration monitor controls replenishment. Toner monitors may take several forms, including optical sensors, capacitance sensors, resistance sensors, inductance sensors, magnetic sensors, etc.
Commonly U.S. patent applications Ser. No. 116,200, now U.S. Pat. No. 4,797,704 filed by L. A. Hill and M. E. Jacobs on Nov. 3, 1987 and Ser. No. 215,971 filed by K. A. Arnold, L. A. Hill, and K. S. Robinson on July 7, 1988 disclose a replaceable development station for developing electrostatic images. The device is slid endwise into place in an electrostatographic machine so that the entire development station is removable when its original supply of toner is exhausted.
Generally, such development stations are disposable. Therefore it is not practical to provide a toner concentration monitor in each station. However, for toner monitors to work optimally, they must not be separated from the development mixture by thick development station walls. However, structural integrity of the development Station requires substantial wall thicknesses. By the present invention, structure has been provided to permit location of a toner concentration monitor on the electrostatographic machine so that the entire development station is replaceable without replacement of the monitor, and which overcomes the problem associated with the thickness of the station walls interfering with the sensitivity of the monitor.